We can extrapolate a large amount of information about what a person thinks or believes, purely by observing their behaviour. There are separate systems in the brain that decode what action is being performed and why that action is performed. Independently, these systems are reasonably well understood but the way in which they interact is still an open question. In this thesis I investigate how we come to understand others actions, particularly if they are unusual or irrational. Irrational actions provide a special test case for examining this question because full comprehension requires an understanding of what an agent is trying to achieve as well as an understanding of why they are performing the action in an unusual way. I examine this question in a series of experiments with typically developing and autistic participants. First, I demonstrate that dissociable brain networks are engaged during irrational action observation using fMRI. I then use eye tracking to examine the cognitive processes that these neural networks reflect in both typical and autistic adults. I report typical cognitive processing of irrational actions in individuals with autism, despite previous reports of atypical neural activity during the same task. Finally, I aimed to examine the social response to irrational actions in typical and autistic children using an overimitation paradigm. I demonstrate that overimitation in typically developing children is socially driven and dependent upon rationality comprehension. However, my data also indicate that children with autism are immune to this drive. I conclude this thesis with a new model of rationality comprehension which links brain, cognition and behaviour. I also propose that individuals with ASC have the cognitive ability to understand irrational actions but may have difficulty with social modulation of their behaviour.